Conventionally, in antistatic rubbers, there have been proposed compositions using a polyether-based compound as an antistatic agent (JP-B 62-15584).
However, the compositions using the polyether-based compounds have a problem in that the cured products thereof have a white turbidity because of a poor compatibility between the silicone polymer and the polyether-based antistatic agent.
Therefore, it is difficult to use the composition in silicone rubber materials used for: a cover of a portable music player, portable game, cellular phone, controller of a game playing device and the like; a silicone rubber mold used for producing a duplicate with a urethane resin and the like; silicone rubber coating on fabrics, and the like, all of which require a higher transparency in the use thereof.
Furthermore, the compositions using the polyether-based compound also have a problem in that the polyether-based compound thermally decomposes at a high temperature, and thus sufficient antistatic effects cannot be exhibited.
In addition, a manufacturing method of heat-curable silicone rubbers has, for example, a post-curing process for 4 hours at 200° C. after a primary curing process, in order to further accelerate the removal of decomposed products of the curing agent or the curing.
There has been proposed that a lithium salt is blended as shown in JP-A 2006-225422 as a technique for imparting a high heat resistance.
It is important for the antistatic products to exhibit antistatic effects on their surfaces, but since the lithium salt is in a state of a solid or powder, there is a problem in which it takes time until the lithium salt exhibits the antistatic effect. Furthermore, the lithium salt has a quality problem in which differences in the dispersion state are easily caused and in which a stable quality is unlikely to be obtained. Therefore, although this problem can be avoided by increasing its amount to be added, there occurs a problem in which the heat resistance and compression set property which the silicone rubber inherently has are remarkably deteriorated.
It has been numerously proposed that an ionic liquid is added to a rubber/plastic composition. Examples of addition to the silicone rubber are in JP-A 2005-298661 and JP-A 2006-83211.
However, a dimethyl silicone polymer is non-polar and has a low solubility constant (SP value) of 14.9 (MPa)1/2, and thus has a low compatibility with the ionic liquid which has an excellent compatibility with a polymer having a high SP value, such as urethane, epichlorohydrin, acrylonitrile butadiene or chloroprene. Therefore, it is difficult to stably add the ionic liquid to the silicone rubber. Even if the ionic liquid is added to the silicone rubber, most thereof bleeds immediately on the surface of the rubber, and the effect cannot be maintained.
Furthermore, in the proposals described above, the addition of the ionic liquid to the silicone rubber is not the addition to the heat-curable silicone rubber, but to a condensation curable system in which room-temperature curing is carried out.
There has been known an ion-conductive rubber composition in which a polyether-modified organohydrgenpolysiloxane and an ion-conductive compound are added to a polyorganopolysiloxane (JP-A 2005-344102).
However, with respect to this composition, the combination with the polyether-modified organohydrgenpolysiloxane is essential and is limited to addition reaction. In addition, there is no description of a polymerization method of the organopolysiloxane.
As the method to solve these problems, there has been proposed in WO-A1 2009/084730 and WO-A1 2009/084733 a method in which a very small amount of a specific ionic substance is added to a heat-curable silicone rubber. However, when increasing the blending amount of the specific ionic substance in order to obtain a higher antistatic property, although the antistatic property is reliably enhanced, there is a case where the inherent properties of the silicone rubber cannot be maintained, for example, a problem of degrading heat-resistance or the like is caused.
In JP-A 2011-201951, there is described a silicone rubber composition including (A) an organopolysiloxane having at least two alkenyl groups in one molecule, (B) a reinforcing filler, (C) an ion-conductive antistatic agent, (D) an alkali metal hydroxide, or the like, (E) an organoperoxide curing agent.
In the composition, there is no description of a polymerization method of the component (A), and since the alkali metal hydroxide (KOH in Example 1) is contained as the (D) component, there is a risk that the polymer chain is cleaved.